1. Field of the Invention
The present invention relates to image processing technology which performs designated processing on normal orthogonally transformed images such as JPEG (Joint Photographic Experts Group) images.
2. Description of the Related Art
Printers and the like are well known as output devices for things such as images downloaded from a network, images shot using a digital camera, and image created using a computer. Printers print images by forming dots on a print medium using ink, etc., so normally, only two gradations are expressed, dot on or off, for each pixel. Thus, to express the gradation that an image has using dot dispersion, so-called half-tone processing is used on the image to be printed.
Meanwhile, in recent years, most image such as images downloaded from a network or images shot using a digital camera are data compressed images, and most of these are JPEG images. JPEG (Joint Photographic Experts Group) is a still image compression method determined by ISO (International Standardization Organization) and ITU-TS (International Telecommunication Union-Telecommunication Standardization Sector) and used for full color or gray scale, and a key feature is that despite the high level of compression, the decrease in image quality is minimal.
Therefore, for example, when printing a JPEG image downloaded from a network using a printer, it is necessary to use half-tone processing. In such a case, in the past, first the downloaded JPEG image would be expanded on a computer, and then half-tone processing of the expanded image would be done, after which it would be sent to the printer.
Specifically, JPEG images are images that have undergone discrete cosine transformation, so when a JPEG image is expanded, reverse discrete cosine transformation is done on that image, and after expanding to spatial coordinates, half-tone processing is done on that expanded image. As half-tone processing, for example, when using the dither method, a dither matrix is introduced for the organizational dither method, and as a threshold value for each matrix, a comparison is made with the corresponding coordinate point pixel value (pixel concentration), and this is binarized.
However, when the conventional type of method is used, with half-tone processing, in comparison to a huge data volume image being expanded to the spatial coordinates, binarization is done while controlling the coordinate position and corresponding matrix value for each pixel successively, leading to the problem of an excessive burden being placed on the CPU, and processing time being very long.
Also, by performing half-tone processing, the image data volume is again increased, and to print using the printer, when a print control command is added to that image, the data volume increases yet again. Therefore, for example, when the printer is connected to a computer via a network or a public line, etc., it is necessary to send this huge data volume image to the printer from the computer via a network or public line, which causes the problem of having the communication time being long, and leading to traffic problems.